1. Field of the Invention
The present invention relates to solenoid valves. More specifically, the present invention is directed to a control circuit for a proportional solenoid valve.
2. Description of the Prior Art
Proportional solenoid valves are available in the form of direct lift-type valves. Such valves are used in applications requiring valves that are voltage controllable and not simply "on" and "off" poppet-type valves, i.e., the valve plunger position must be controlled by the magnitude of the solenoid input voltage. Typically, the greater the input voltage, the greater the plunger lift and, consequently, the greater the flow rate or downstream pressure produced by the valve. The valve must be specially designed designed to perform this proportional function, and the solenoid drive circuit must be adapted to meet the unique electrical characterstics of such a valve. For example, the heating of the valve while attempting to maintain a plunger position produces an increase in the coil resistance which increase, for the same input voltage, decrease the solenoid current. The current decrease reduces the magnetic force and allows the plunger to drop to ultimately reduce the flow rate or downstream pressure. Such an undesired decrease in the plunger lift is effective to produce an erroneaous valve operation which results in an undesired flow rate or downstream pressure. Further, such solenoid drive circuits have been continuous drive arrangements which simply maintain a continuous solenoid drive current at a value theoretically resulting in the desired plunger lift. Such a continuous operation exacerbates the aforesaid heating problem of the solenoid coil as well as wasting electrical energy during the valve operation.
Accordingly, it would be desirable to provide a control circuit for a proportional valve which would maintain a desired plunger position by compensating for such a resistance change of the solenoid coil during the operation of the valve and minimize electrical power consumption during a valve operation.